If you only want to play Minecraft and browse the web, this partitioning will do:
/dev/sda = /
/dev/sdb = unused
Alternatively, you could make a RAID 1 volume of the two disks to protect against the failure of one drive. A RAID 1 volume uses two (or more) disks with identical contents, so if one disk fails, you still have a working system. Or you could use the second drive to back up your data.
/usr is not useful. On a typical desktop machine,
/usr makes up the bulk of what is not
/home. If you have little data on
/home, with your scheme, you'd end up with
/dev/sda almost empty. If you have a lot of data in
/home, make it
If you need more than 8GB for the system, there's no particularly meaningful place to break it up.
/usr/lib would be a good bet. Anyway, you can mount
/dev/sdb wherever you like, and move one or more directories there and create a symbolic link.
Rather than pick a mount point, there's a simpler approach to splitting your system between two drives: make it a RAID 0 volume. RAID 0 combines two or more disks into one, in an arrangement that tries to spread the load evenly between the disks. The advantages of RAID 0 over a mount point is that you don't have to worry at all about choosing the mount point right for splitting the space evenly, and that it's a bit faster. The downside of RAID 0 is that if one of the disks fails, the whole filesystem will be unusable.
To install Ubuntu on a RAID volume, you'll need to use the server (alternate) installation media and do manual partitioning.